The braking pressure results from the pedal force and the auxiliary force, which is produced from the pressure difference between the working chamber and the vacuum chamber (low-pressure chamber) of the vacuum brake booster. The auxiliary force component is constantly increasing corresponding to a boosting factor defined by construction until the point of maximum boosting. The maximum pressure difference is reached in the point of maximum boosting. A further rise of the output force is only possible by an unusual increase of the brake application force, in particular the pedal force. The point of maximum boosting lies at a defined hydraulic pressure (maximum boosting pressure) of a master brake cylinder succeeding the vacuum brake booster.
The term ‘brake control characteristic’ herein and in the following implies a characteristic curve of a vacuum brake booster which represents the correlation between a vacuum or low pressure prevailing in the vacuum chamber or low-pressure chamber of the vacuum brake booster and a resulting brake pressure or a brake force (maximum boosting pressure or maximum boosting force) being at their maximum under the given conditions.
A driving motor (internal combustion engine) generally supplies the vacuum for the vacuum brake booster. If the maximally attainable boosting force is insufficient due to an only weak vacuum supply, which is a more frequent case in novel engine technology such as a direct gasoline injection system or Diesel engine, there is need for an additional brake boost. One possibility of producing an additional brake force or an additional brake pressure involves the use of ‘active’ hydraulic brake boost. This is achieved e.g. by means of a hydraulic pump. The hydraulic pressure, which results in the hydraulic master brake cylinder from the brake force that is introduced by the driver by way of the brake pedal and boosted by means of a vacuum brake booster, is additionally increased by the hydraulic pump. Said pump is driven by an electric motor actuated by an electronic brake control unit.
It is necessary in systems of this type to determine the brake control characteristic and the point of maximum boosting of the vacuum brake booster. The reason is that this point determines the point of transition between the brake boost by way of the vacuum and the assistance by way of the hydraulic pump. The brake control characteristic and the point of maximum boosting or maximum boosting pressure depend on various parameters such as the membrane surface of the vacuum brake booster, vacuum level and master cylinder pressure. Due to e.g. tolerances of the components concerned, it is problematic to fix a sufficiently precise brake control characteristic and a precise point of maximum boosting or maximum boosting pressure.
It is known in the art to employ pressure sensors in/on vacuum brake boosters to sense and evaluate a pressure difference between the low-pressure chamber or vacuum chamber and the working chamber.
DE 44 36 297 C2 discloses a vacuum brake booster and a method of operating a vehicle brake system, wherein the detected pressure difference is evaluated for the actuation of a control valve in the vacuum brake booster.
DE 197 29 158 C2 discloses a vacuum brake booster with a control unit carrying two pressure sensors with two air ducts that extend into the low-pressure chamber and the working chamber of the vacuum brake booster. The control unit is used to control the vacuum brake booster.